Phosphorus-31 nuclear magnetic resonance and left ventricular pressure development (dP/dt) were used to test the hypothesis that age-related differences in myocardial functional recovery after ischemia and cold crystalloid cardioplegia (CCC) are the result of an inverse relationship between recovery and the decrease in intracellular pH (pH_i) during ischemia. Neonatal (3-8 days) and adult rabbit hearts were Langendorff perfused using two protocols: (1) control – 30min perfusion, 30 min global ischemia, 2 h reperfusion; (2) CCC – the same except ischemia was initiated after a 4-min infusion of cold hyperkalemic solution. Analysis of variance and the Tukey test showed the following significant differences between the protocols (p < 0.05). CCC decreased inorganic phosphate (Pi) during ischemia in both age groups, but more in adult hearts, and decreased Pi during reperfusion in adult hearts. CCC increased pH_i during ischemia and ATP during ischemia and reperfusion in both age groups but more in adult hearts. CCC increased dP/dt during reperfusion only in adult hearts. The results are consistent with the hypothesis.